There has been a recent surge in interest for continuous-wave, solid-state yellow laser sources, for a diverse range of applications including ophthalmology, biomedicine, guide stars and visual display. Currently, CW dye lasers and krypton ion lasers offer CW output powers of up to 1 W. Recently, frequency doubled neodymium lasers have become available, typically producing 100 mW output at 563 nm wavelength. Among the recently reported approaches to demonstrating CW laser sources are a 40 mW, 575 nm frequency doubled Yb fibre laser, a 1.52 W, 590 nm frequency doubled Raman fibre laser pumped by a diode-pumped Yb fibre laser, and 0.7 W, 593.5 nm laser source based on sum frequency mixing of the 1064 and 1342 nm lines of two Nd:YVO4 lasers.
Crystalline solid-state Raman lasers have attracted considerable interest in the past few years as a means of increasing the versatility of fixed wavelength infrared gain media, in particular as a means to generating multi-watt visible wavelengths in the 500-600 nm spectral range with high conversion efficiency, up to 8% with respect to the diode pump power. These have provided resonator designs which are very straightforward to implement. This has been achieved for example by intracavity Raman-shifting of a Nd:YAG laser in KGW with intracavity frequency doubling in LBO. Alternatively, using a simple extracavity arrangement, Raman shifting in KGW of a high peak power green laser at 532 nm has been used to generate wavelengths between 555 nm and 658 nm.
To initiate the Raman process these systems have only operated in the pulsed regime (typically Q-switched with ns pulses) as the Raman threshold was considered too high for cw operation. Recently continuous wave Raman shifted operation in Ba(NO3)2 has been demonstrated using an external resonator with a Ar+ pump source resulting in a 1st stokes power of 164 mW at the visible wavelength of 543 nm [see A. S. Grabtchikov et al., Optics Letters, 29, 2524 (2004)]. Also a diode-pumped self-Raman converting laser based on Nd:KGW has been reported, that produced 54 mW (at 1181 nm) [see Demidovich, et al. “Continuous-wave Raman generation in a diode-pumped Nd3+:KGd(WO4)2 laser” Optics Letters, 30, 1701-1703 (2005)]. Thresholds as low as 1.15 W were recorded. Output powers of about 800 mW have recently been reported from a intracavity Raman shifted Nd:YAG/KGW laser using a simple diode end-pumped configuration. None of these continuous wave Raman lasers have incorporated intracavity frequency doubling, nor provided any indication on how such intracavity frequency doubling could be accomplished.
There are however very few solid-state continuous-wave (CW) lasers which operate at yellow wavelengths. There is therefore a need for an efficient and practical system for generating CW laser output.